vrtcl is a record-centric data format that seamlessly integrates non-record structures, namely lists. It supports type indicators for values, but makes it easy to avoid them when they aren't wanted or needed. The goal of vrtcl is to be more readable and concise than other formats without sacrificing expressivity, and also to be extensible.

vrtcl was conceived as an alternative to JSON, which bills itself as a "fat-free alternative toXML". Although JSON is lean, Tcl syntax is leaner and cleaner. Other Tcl formats such as huddle and TDL exist, but vrtcl is different in some key characteristics.

Scripts

A comment whose first non-whitespace character is ! is a directive. The following directives are built-in:

PUSH

the next command is interpreted as a configuration record supplying that applies to the remainder of the record.

POP

revert to the previous configuration

Built-in Command Types

field or fld or f

A command composed of at most three words, as described in Fields.

cell or cel or c

A command composed of one word which is the value, or composed of two words which are the configuration record and the value, respectively.

Fields

The number of words in a field determine the interpretation of the words:

one

The word is the value.

two

The first word is the name of the field, and second word is its value.

three

The first word is the name of the field, the second word is the configuration record for the field, the third word is the value. A configuration record is the second word rather than the third to facilitate parsing a field incrementally from a stream without reading the entire value first.

Values

The type of a value may be specified by an applicable configuration record. If a value is a Tcl dictionary and its type is not otherwise specified by an applicable configuration record, it is a record. If a value is not a record and its type is not specified by an applicable configuration record, the value is not interpreted. The built-in types avaiable for use in configuration records are:

A computed value. The given value is used by the system to compute the final value.

References

A reference is a record that locates another command or sequence of commands in the record hierarchy. If a reference is a script substitution, the system executes the script to resolve the reference. Otherwise, the value is a system identifier for another command.

Configuration Record

A configuration record takes effect prior to processing of any value it accompanies, and specifies, among other things, the interpretation the value. A configuration record that accompanies a PUSH directive is in effect for the remaining commands in the record.

If the first command of the configuration record contains only one word and that word is a number as understood by expr or if the entire configuration record consists of only one field and the value is not an type known by the system, that value is a system identifier for the value that the configuration record accompanies. Otherwise, if the first command of the configuration record contains only one word, it is a field whose name is type, and whose value is the word. If the second command contains only one word, it is a field whose name is ctype and whose value is that word.

The built-in configuration fields are:

The built-in fields of a configuration type field are:

ctype

The default type for commands in the record.

contains

a record in which fields identify which elements can occur in the value, along with constraints on its the occurrence.

The built-in fields of a configuration elements record are

type

Default Configuration record

What follows is the default configuration record for a record. A vrtcl interpreter is initialized with this configuration. The dictionary indicates the names of fields that may appear in the configuration record, and what they signify.

dictionary {
ctype {
description {
The type of the items in a container such as a record.
}
}
type {
description {
The type any value the configuration record is applied to.
}
}
types {
description {
A record in which each field is a describes the types of values of items
in the record The name of each field is the name of the described
`type`.
}
}
id {
description {
A system identifier for the field this record is part of.
}
}
ref {
description {
A reference to another record, which is to be considered the actual
value. In this case, the value for the current record is simply a
presentational label for the referenced value.
}
}
triples {
description {
If true, indicates that all commands in values of type "record"
contain exactly three words. This allows the parser to efficiently
process records in a stream. Commands must then contain three
words.
}
{
}
type record
ctype fild
# Except for record, whose default type is "field", all other values are by
# default implicit.
directives {
PUSH {
}
POP {
}
}
types {
cell {
type record
alias c
}
field {
type record
alias f
}
list {
alias l
}
number {
alias {num n}
}
null {
alias {nul u}
}
record {
alias {rec r}
}
referene {
alias ref
}
value {
alias {val v}
}
}

#! /bin/env tclsh
#! PUSH
#This is a vrtcl document
{
types {
#change the `ctype` of record to `cell`
record {
ctype cell
}
}
}
#the ctype for a record is now "cell", so there are no record names
list {one two {three four}}
#also valid, but it isn't explicit that the value is a list:
{one two {three four}}
rec {
name Capaneus
son Sthenelus
}

One advantage of vrtcl over huddle is that there is no need for a keyword like HUDDLE in the data. vrtcl records can be composed of other vrtcl records. Values that look like vrtcl data can be given the value or list types, so that they are not interpreted as part of the structure of the vrtcl record.

In huddle, the type of each value is explicit in the notation. vrtcl relies on its rules to obviate the need for explicit type notation in the common cases.

In the Tcl examples above, the format is that of a script where each command takes exactly one argument and knows the type of that argument. Even though JSON expresses types in its syntax, JSON data is usually also consumed by a script which already knows what type to interpret the data as, since it conforms to a scheme that is usually documented as part of a service API. In most cases, the additional type syntax of JSON is not needed.

The first type information that comes to mind for the Tcl format is the ability to distinguish between a word and a list. One approach in the spirit of Tcl would be to interpret all values as lists, making sure that where a single value is intended, it is formatted as a list containing one item:

This is already a reasonable format to work with. A more full-featured notation would support type information and other metadata in an extensible way. In JSON, it is possible to differentiate between the string "3.14159" and the number 3.14159, but all other numeric types are inferred by the format of the number. This is a hint that explicit typing is often not needed. In JSON, this numeric type inference stems from from the origins of JSON in Javascript, which only provides one numeric type. Given that JSON data is typically described in the documentation for an API, the explicit double-quote syntax to differentiate numbers and strings is in practice probably always redundant with the documentation, and therefore not really necessary. Where type information is used by the consuming program, it would probably always be trivial (excepting of course political issues such as backwards compatibility and organizational inertia) to add a field to the API to specify the type of the value, rather than relying on the syntax of the serialization format.

[TODO: Find real-world examples of programs that actually rely on this distinction]

The Tcl examples above are valid scripts in which each "command" takes exactly one argument. Where explicit typing is desired, a variation could be employed in which a command given only one argument treats it as its list value, and when it is given two arguments, treats the first argument as metadata, and the second as the value:

AMG: Is this called vrtcl or vertcl? Both spellings appear on this page. Do these two terms refer to distinct yet related concepts?

PYK 2014-08-03: I was playing with both names, and I guess my mind hadn't settled on one. I'm going to go with with "vrtcl", which will entail changing the name of this page. I just changed all spellings on this page to "vrtcl".

AMG: "Each field is a command whose semantics depend on the number of words in the command:" Customary Tcl terminology counts the name of the command as the first word of the command. Here, you probably mean to say arguments instead of words.

PYK 2014-08-03: the vrtcl specification avoids the use of "arguments", using "words" instead, and "first word" refers to the word in the position that normally names a command. This aligns with rule 2, which defines the name of the command as the first word in the command. I note though that rule 2 is ambiguous or even contradictory, saying that the first word is used to locate a command procedure, but then immediately stating that all words of the command are passed to the procedure, which is of course not true if the command name is considered one of the words of the command. Perhaps rule 2 should be ammended to state that "all of the remaining words are are passed to the command procedure".

PYK: Yes. TDL riffs on XML, supporting attributes as its metadata feature, but vrtcl provides a full isomorphic metadata mechanism.

AMG: I have no JSON experience, so I would very much like to know whether it's possible for a program to make decisions based on the type of a value. This is essentially the same question as whether or not explicit type tagging has any value. You argue that it does not (EIAS and all that), but then proceed to present a way to incorporate type tagging, with only a TODO note to research whether or not there's any reason to do so. Perhaps someone else could fill us in.

Like you, I argue that inline type tagging is not useful because the consumer already knows what types to expect. In other words, the schema is embedded in the program itself.

That's not to say explicit schemas aren't useful. They are good documentation, they make it possible for a generic program to validate a document, and they may assist compression. But merely embedding types in the document does not constitute a true schema. It cannot protect against misspelled field names, and it fails to document data relationships and unused features. So explicit schemas really ought to be external documents. In short, I see no benefit to embedding type tags in this application.

Well, actually, I do see one benefit, and maybe this is what you had in mind. Since other formats require type tags, and since it's generally impossible to unambiguously detect the "type" of a Tcl word, compatibility with other formats requires type tagging. That type tagging can go inline or in an explicit schema.

As you may guess from what I said about schemas, I prefer the latter approach, though I freely admit that the former is much easier to implement. There is a third approach, which is to write a program that implicitly embeds a schema and whose purpose is conversion. This surely works, but I think of it as a one-off sort of solution, whereas this page is dedicated to describing a general approach, so again I think an explicit schema would be more appropriate.

... I hate to say it, but there is a fourth way to go. PLEASE do not use this. [tcl::unsupported::representation] tells you what C-based type the script most recently wanted any given value to be. Again, don't do this, since it will result in a very brittle and unpredictable program, requiring the end user to do anti-Tcl contortions to get the desired results, and even then with surprises. The only reason I mention this at all (aside from taking the opportunity to warn against it) is because a real-world Tcl extension does it: tcom. And it is sometimes very shocking [1].

AMG: What is the point of applying an extra level of list encoding to each command's argument? It's already a single word on account of being one argument. I don't see how this helps in disambiguating its type, if that's even a goal (see above).

PYK 2014-10-15: the default types have been tuned since AMG made this comment, so the extra level of braces doesn't show up as much. It still can, depending what types one sets for a value. More detail below, and through the spec and examples.

PYK 2015-06-07: At the point the syntax and semantics of vrtcl have changed a bit, and the extra levels of list encoding have all dropped out of the examples.

AMG: The format you present is script-oriented, meaning that you use newlines as field separators. Do you also allow semicolons? How about substitutions? Comments? Double quotes instead of braces? Backslashes? Blank lines? Is whitespace significant, e.g. indenting? What about non-data commands such as looping and conditionals, such as those I demonstrate in Config file using slave interp?

Except for those showing type tagging, all the examples you present could instead be viewed as dicts, since newlines work as list/dict element separators just as well as any other form of whitespace. What benefit does your approach have over nested dicts?

Some of the rules and examples have changed overnight, making some of AMG's comments appear a bit non-sequitor. Sorry about that.

The Amazon DynamoDB API Reference selects a comparison mode based on the type of AttributeValueList, so it's certainly possible for a program to react to the type of a value, but it would also be trivial to communicate the type as the value of an additional field, perhaps AttributeValueListType, which obviates the need to have type indicators built into the syntax. vrtcl is very much about clean syntax, and some of the complexity of the ruleset arises in order to minimize syntax and maximize conciseness and legibility.

vrtcl supports type indicators in order to support lossless tranformations to/from other formats, such as XML and JSON, and also because even though type indicators are often not needed, maybe sometimes they are, and maybe if vrtcl supports them, they will be put to good, judicious, and even ingenuitive use.

vrtcl aspires to be extensible like XML is, and JSON is not. If the type tagging is removed from the lexical level but supported in the grammar, additional types can more easily be added. The configuration record is also wide open to other novel use.

I fully agree with your analysis of the usefulness of schemas, and the elements configuration field is a hint at the direction vrtcl will take in support of schemas. So why also support type tags in record fields? Because schemas can be a hassle to write, and making them mandatory would be a barrier to usability in many cases. Even if a programmer has an idea of what the data schema is, it can take a while during development for the schema to settle down. In the meantime, the programmer often just wants to throw data around and work with the data in an ad-hoc fashion for a while. vrtcl aims to be a superior alternative at all stages. Besides, if both schemas and inline configuration are available, interesting patterns might emerge. In short, the reasons are what you guessed I had in mind, and then some.

Supporting configuration at each field could end up giving schemas a CSS characteristic, where schemas for subdocuments are built up in a cascading manner. configuration is a vrtcl document in its own right, giving it a flexibility that XML, with its special syntax for attributes, doesn't have. How far down the configuration rabbit hole will people go? vrtcl leaves that undefined to keep the possibilities open.

PYK 2014-10-15 2016-08-08: vrtcl defaults have recently been tuned to reduce the brace count. Extra braces usually happen now in the context of a list of lists. In the example below, three four is the first item in the third list:

example {list; list} {one two {{three four}}}

If a field contains only one word, that word is the value of the field, and is interpreted as a list. This rule minimizes the number of characters necessary convey the structure in some common cases. The idea is that the programmer doesn't want to be bothered with naming the field or providing any configuration. In most cases, a list value will be exactly what's wanted, or at least it isn't that difficult to format a value as a list containing one item.

At points where it feels like there are too many braces, vrtcl probably supports some syntax that will eliminate those extra braces. In the previous example, The default ctype of a list list is also string, meaning that it can be written like this:

example {one two {three four}}

Regarding semicolons, yes, they have their normal meaning. Some of the examples already illustrate semicolons, and a few examples contain comments. Regarding substitutions, double quotes instead of braces, Backslashes, blank lines, whitespace, and indenting: Yes! I haven't ruled any of these things out for vrtcl, and I hope it can continue to support all of them. All normal Tcl processing will be performed on the words of a field prior to its evaluation by vrtcl. Tcl syntax makes those things easy enough to avoid when they are unwanted. But this is where the embryonic status of vrtcl comes into play. Any of these features that turn out to be too distracting or unwieldy, might still get cut, though I don't anticipate that. As is the case with standard Tcl, substituted values will not be rescanned, but macros such as those found in Config file using slave interp could be implemented in a more controlled manner through the configuration "hook".

I'm not currently considering adding explicit general scripting capability in the form of looping, conditionals, or other programming features. vrtcl is, after all, a data format. I think the XSLT approach, in which a separate document is created that describes transformations to effect on other documents, makes for a good separation of concerns. The configuration records of vrtcl turn it into the swiss cheese of data formats, so there's plenty of room to fill in the blanks as occasion arises.